(1) Field of the Invention
The present invention relates to an improvement in the carburetor of a spark-ignition internal combustion engine. More particularly, the present invention relates to an air valve type two-stage twin compound carburetor having an air valve on the secondary side.
(2) Description of the Prior Art
In a spark-ignition internal combustion engine provided with a carburetor, a phenomenon called "running-on" sometimes takes place. This occurs when the combustion chamber of the engine is hot and a large quantity of an air-fuel mixture is present in the carburetor. Fuel vapor is generated because of the high temperature of the fuel. The rich air-fuel mixture is supplied to the combustion chamber from a primary idle port or a low-speed port after the engine ignition switch has been turned off, but while the engine is still being rotated by the force of inertia. Even though the ignition is off combustion occurs due to natural ignition and rotation of the engine is continued. This phenomenon is called "running-on". When the throttle valve is not smoothly returned to the closed position due to the presence of foreign matter or the like, a negative pressure is produced in the main nozzle, and fuel is injected, causing a large quantity of an air-fuel mixture having a rich air-fuel ratio to be supplied to the engine. This sometimes causes the phenomenon of "running-on" to be continued for a long time. Combustion by such a "running-on" phenomenon is incomplete combustion, and if the "running-on" is continued for a long time, in the case of a vehicle having an exhaust gas-cleaning catalyst attached to an exhaust pipe, the catalyst is abnormally heated by the reaction of unburnt gas on the catalyst and fusion loss of the catalyst is likely to occur. Moreover, this "running-on" phenomenon is not preferred from the viewpoint of the fuel economy.
In the conventional carburetor, a valve member for opening and closing a fuel passage of the carburetor is arranged as a means for preventing the occurrence of the running-on phenomenon, and the fuel passage is closed, simultaneously when the ignition is turned off, to stop the fuel from being supplied to the engine. An electromagnetic valve is often used as the valve member for opening and closing the fuel passage. However, the electromagnetic valve is relatively expensive, and, if the electromagnetic valve is attached to the fuel passage, when the electromagnetic valve gets out of order, the driving characteristics of the vehicle are often degraded. This is one disadvantage of using such an electromagnetic valve.
In a twin compound carburetor having a primary system and a secondary system, a fuel nozzle is sometimes arranged in the secondary low-speed system, so as to cope with a bad return of the secondary throttle valve, which is likely to occur. Furthermore, in an air valve type twin compound carburetor, in which an air valve is arranged in an air introduction zone above a small venturi of the secondary system and a closing moment is imposed on the air valve, the negative pressure of the secondary system is increased by the choke effect of the air valve to increase the injection response of the fuel. In such a carburetor, if, on turning ignition off, closure of the secondary throttle valve is hindered, for some reason or other, fuel is caused to flow out of the fuel nozzle by the choke effect of the air valve, and running-on is promoted by the thus enriched air-fuel mixture. Therefore, in this air valve type carburetor, it is necessary to prevent the fuel from flowing out in the secondary main system simultaneously with the ignition being turned off, and therefore, the structure of the carburetor inevitably becomes complicated.